The present invention relates generally to crosslinkers useful in preparing crosslinked polymer compositions and further compositions therefrom.
Polymers are generally crosslinked to increase the cohesive strength, rigidity, heat resistance, or solvent resistance of a polymer composition. However, the use of certain crosslinkers can make it difficult to apply the compositions to a substrate in, for example, the form of a coating. Certain compositions that are hot-MELT processable materials must have a sufficiently low viscosity upon melting, such that they can be readily hot-melt processed (e.g., applied to a substrate). The presence of crosslinks in a material generally increases the melt viscosity of the material, many times making it impossible to hot-melt process the materials.
In an attempt to provide materials having sufficient cohesive strength and/or rigidity, as well as processability, thermally reversible crosslinks have been used. It is well known to incorporate thermally reversible crosslinks into polymers. Upon heating, the crosslinks dissociate or break. Upon cooling, the crosslinks reform. This sequence can be performed repeatedly. Thermally reversible crosslinks find many uses, for example, in hot-melt processable and re-moldable (or recyclable) materials. By incorporating thermally reversible crosslinks into polymers, a composition can be heated to form a coating or mold from the composition and then return to its original crosslinked state.
For examples of thermally reversibly crosslinked polymers, see U.S. Pat. No. 3,435,003 (Craven); U.S. Pat. No. 4,617,354 (Augustin et al.); and U.S. Pat. No. 5,641,856 (Meurs). Also see, PCT Publication Numbers WO 95/00,576 (Heyboer) and WO 99/42,536 (Stark et al.), as well as Canary et al., xe2x80x9cThermally Reversible Crosslinking of Polystyrene via the Furan-Maleimide Diels-Alder Reaction,xe2x80x9d Journal of Polymer Science Part A: Polymer Chemistry, Vol. 30, pp. 1755-9 (1992) and Chujo et al., xe2x80x9cReversible Gelation of Polyoxazoline by Means of Diels-Alder Reaction,xe2x80x9d Macromolecules, Vol. 23, pp. 2636-41 (1990).
Further examples of thermally reversibly crosslinked polymers are thermoplastic elastomers, such as those described in Kirk-Othmer Encyclopedia of Chemical Technology, 4th Edition, Wiley: 1994, Vol. 9, pages 15-37. Such thermoplastic elastomers have many of the physical properties of rubbers (e.g., softness, flexibility, and resilience), but, in contrast to conventional rubbers, they are hot-melt processable. With such thermoplastic elastomers, the transition from a processable hot-melt to a solid, rubberlike composition is rapid, reversible, and takes place upon cooling.
Such thermoplastic elastomers are often multiphase compositions in which the phases are intimately dispersed. In many cases, the phases are chemically bonded by block- or graft-polymerization. At least one phase consists of a material is that is relatively hard or glassy at room temperature, but becomes rubbery upon heating. Another phase consists of a softer material that is rubberlike at room temperature (i.e., an elastomer). For example, a simple structure of a multiphase thermoplastic elastomer is an A-B-A block copolymer, where A is a hard phase and B is an elastomer (e.g., poly(styrene-b-elastomer-b-styrene)). Examples of these materials are included in, for example, U.S. Pat. No. 3,639,517 (Kitchen et al.) and U.S. Pat. No. 4,221,884 (Bi et al.), and Japanese Patent Publication Number 52[11977]-129795. References describing the use of these materials in formulating adhesives include, for example, U.S. Pat. No. 4,444,953 (St. Clair); U.S. Pat. No. 4,556,464 (St. Clair); U.S. Pat. No. 3,239,478 (Harlan); and U.S. Pat. No. 3,932,328 (Korpman).
It may be desired, however, to provide a composition that is chemically and/or physically different (e.g., having different adhesive properties) after application to a substrate as compared to the composition prior to its application. For example, U.S. Pat. No. 5,322,731 (Callahan, Jr. et al.) describes adhesive beads comprising a pressure-sensitive adhesive (PSA) core and a discontinuous organic polymer shell that makes the bead essentially non-tacky at room temperature. Upon application of heat and/or pressure to the bead, the core and shell materials can be blended to form a resultant PSA. By providing compositions that are essentially non-tacky before application, easier handling of the compositions is facilitated. That is, conventional handling and coating equipment, such as hopper feeders, powder conveyers, and hot-melt stick applicators can be used, without requiring specialized handling and coating equipment.
However, when using the adhesive beads described by Callahan, Jr. et al., the core and shell materials must be selected both such that they are respectively tacky and non-tacky to the touch at room temperature and such that, upon application, they combine to result in a composition having PSA properties. Furthermore, when the tacky core materials are uncrosslinked, the beads tend to be deformable, especially under pressure or at high temperatures. The shell could potentially rupture due to this deformation, causing the beads to prematurely agglomerate. As another example, see U.S. Pat. No. 5,804,610 (Hamer et
As an alternative to the multi-layer constructions of, for example, Callahan, Jr. et al. and Hamer et al., U.S. Pat. No. 3,909,497 (Hendry et al.) describes solid polymers that are thermally degradable to flowable (or at least softened) compositions. The polymers contain heat-sensitive groups that cleave at a temperature substantially lower than that at which the thermal degradation would occur in their absence. Cleavage is effected essentially in the absence of materials reactive with the resulting molecular fragments. The degradation products are useful as adhesives, plasticizers, fillers, etc.
Heat-sensitive groups of Hendry et al. are taught to be azo groups, carbonate groups, ester groups, and amine-oxide groups conforming to specific formulas recited therein. However, azo groups and amine-oxide groups typically fragment into fragments containing free radicals or fragments (containing ethylenic unsaturation) susceptible to free radical polymerization, which are disadvantageously susceptible to recombination and degradation (e.g., affecting weatherability and durability of the resulting composition). The same applies to ester groups and carbonate groups, depending on their particular chemistry. Those of ordinary skill in the art will recognize that fragmentation of ester groups and carbonate groups of hydroxy compounds having active xcex2-hydrogens, as taught by Hendry et al., results in fragments containing ethylenic unsaturation.
PCT Application Number US99/06,007 (Everaerts et al.) describes a base copolymer that exhibits little or no tack prior to its combination with a plasticizing agent. Thus, the base copolymer can be transported and processed without special handling and processing equipment. However, formulation latitude is also compromised with those compositions because the base copolymer is typically a high glass transition temperature, Tg (i.e., a Tg of at least about 0xc2x0 C.), high shear storage modulus (i.e., a shear storage modulus of at least about 5xc3x97105 Pascals when measured at 23xc2x0 C. and 1 Hertz) base copolymer in order for useful pressure-sensitive adhesive materials to be formed when the base copolymer is combined with the plasticizing agent.
Further compositions that are chemically different (e.g., leading to compositions having different adhesive properties) after application to a substrate as compared to the composition prior to its application are desired. It is also desired to provide formulations that are relatively stable after transformation to their altered chemical state, as compared to, for example, the compositions of Hendry et al. that contain free radicals or fragments containing ethylenic unsaturation that are susceptible to recombination or degradation.
The present invention is directed toward compositions that are chemically different after application to a substrate as compared to the composition prior to its application. Advantageously, compositions of the invention are relatively stable after transformation to their altered chemical state.
Degradable crosslinkers of the invention are useful in such compositions. In one embodiment of the invention, a degradable crosslinker comprises at least one energetically labile moiety and at least two free radically polymerizable groups, wherein the degradable crosslinker is capable of fragmentation into at least two fragments upon activation by an external energy source, and wherein the at least two fragments are essentially free of free radicals and ethylenic unsaturation. For example, compositions containing such degradable crosslinkers may be indicative of those in a first chemical state, prior to applying the compositions to a substrate.
A wide variety of degradable crosslinkers of this nature are provided. For example, the crosslinker can comprise at least two energetically labile moieties. The energetically labile moieties may be the same or different. In one embodiment, the energetically labile moiety comprises an ester moiety. For example, the energetically labile moiety may be an amide ester or aldoxime ester moiety.
In preferred embodiments, the crosslinker is storage-stable until activation thereof. Activation of the degradable crosslinker occurs using any suitable energy source. In one embodiment, the crosslinker is a thermally degradable crosslinker, wherein the degradable crosslinker degrades upon activation by a thermal energy source.
The chemical nature of the crosslinker is preferably selected according to the chemical nature of the polymer that it is intended to be incorporated into or that it is incorporated into (in the case of compositions comprising the degradable crosslinker). In preferred embodiments, the crosslinker is a (meth)acrylate degradable crosslinker.
Crosslinked polymer compositions comprising at least one polymer and at least one such degradable crosslinker incorporated into the at least one polymer are also disclosed. In further embodiments, the compositions can comprise at least two degradable crosslinkers incorporated into the polymer. The crosslinked polymer compositions may also comprise at least one non-degradable crosslinker.
The crosslinked polymer compositions can be tacky to the touch or essentially nontacky at room temperature. In certain embodiments, the composition is at least partially tacky at room temperature. Tackiness of the composition is adjustable by tailoring the composition according to the desired application. Certain applications benefit from providing the composition in a free-flowing form. To facilitate this form, the composition may further comprise at least one of a dusting agent and a coating agent.
A method of the invention comprises a method of transitioning a crosslinked polymer composition from a first chemical state to a second chemical state, the method comprising the steps of: (1) providing at least one crosslinked polymer composition described above and (2) activating at least a portion of the crosslinked polymer composition by applying an external energy source to at least a portion of the crosslinked polymer composition to fragment at least a portion of the degradable crosslinker.
In preferred embodiments of this method, the external energy source is a thermal energy source. The step of activating can occur prior to applying the crosslinked polymer composition to at least a portion of a substrate, while applying the crosslinked polymer composition to at least a portion of a substrate, or after applying the crosslinked polymer composition to at least a portion of a substrate.
According to this method, further steps may also be performed. For example, the method can further comprise the step of catalyzing fragmentation of the at least one degradable crosslinker. The method can further comprise the step of applying the crosslinked polymer composition to at least a portion of a substrate. After application to the substrate, the crosslinked polymer composition can be at least partially recrosslinked.
Also disclosed are polymer compositions comprising a polymer and at least two pendant moieties on the polymer, wherein the pendant moieties are the reaction product of fragmentation of the degradable crosslinker and wherein the pendant moieties comprise the at least two fragments that are essentially free of free radicals and ethylenic unsaturation. For example, compositions of this nature may be indicative of those in the second chemical state.
Crosslinkers of the invention are useful for providing a wide variety of compositions that are chemically different (e.g., having different adhesive properties) after activation and, optionally, application to a substrate, as compared to the composition prior to its activation. As a result of a change from a first chemical state to a second chemical state, certain physical properties of the composition are altered.
In one embodiment, the crosslinkers are useful for providing compositions having varying levels of tack and/or compliance, and therefore adhesion, before and after activation of the composition (i.e., fragmentation of at least a portion of the crosslinkers in the composition). The varying levels of tack and compliance may enable tapes, for example, incorporating the compositions to be more easily repositionable and/or removable before activation as compared to after activation. Furthermore, varying levels of tack may facilitate easier handling and processing of the composition.
According to one aspect of this embodiment, a crosslinked polymer composition is essentially non-tacky before activation, but becomes tacky, enabling it to be used as an adhesive (e.g., a pressure-sensitive adhesive), after activation. According to another aspect of this embodiment, the crosslinked polymer composition is tacky before activation, becoming even more tacky after activation. Thus, multi-layer constructions, such as those described by Callahan, Jr. et al. (U.S. Pat. No. 5,322,731) and Hamer et al. (U.S. Pat. No. 5,804,610), are advantageously not necessary with the present invention.
In a preferred embodiment, the crosslinked polymer composition is xe2x80x9cfree-flowingxe2x80x9d before activation. xe2x80x9cFree-flowingxe2x80x9d crosslinked polymer compositions are those compositions containing a multitude of solid state particulates that do not substantially agglomerate (i.e., the particulates are capable of being moved by gravitational forces alone) at temperatures below the activation temperature of the degradable crosslinkers. Free-flowing crosslinked polymer compositions contribute to easier feeding and delivery methods. For example, free-flowing crosslinked polymer compositions facilitate feeding hot-melt coating compositions. Furthermore, shipping and handling associated with free-flowing crosslinked polymer compositions is typically less expensive and more convenient than shipping and handling associated with bulk polymer compositions.
In another embodiment, crosslinked polymer compositions (e.g., adhesives, varnishes, waxes, paints, various coatings, such as protective and decorative coatings, etc.) of the invention can be used to provide compositions having varying levels of solvent resistance or solubility. After activation, crosslinked polymer compositions of the invention tend to be more soluble and less solvent resistant, enhancing their ability to be degraded for easier removability from articles on which they are incorporated. For example, the compositions can be made more soluble (e.g., etchable) after activation, facilitating their use in forming patterned coatings. Furthermore, when used as, for example, repulpable adhesives, the crosslinkers provide compositions having varying levels of solvent resistance or solubility, such that, after activation, the compositions tend to be more soluble and less solvent resistant, enhancing their ability to be degraded for recycling of the articles in which they are incorporated.
In another embodiment, crosslinked polymer compositions of the invention can be used to alter the failure mode of certain compositions. Typically, for example, after activation, adhesive compositions containing crosslinkers of the invention tend to fail more cohesively, as opposed to failing adhesively.
A wide variety of other uses for crosslinkers and crosslinked polymer compositions of the invention will be apparent to those of ordinary skill in the art. Generally, the crosslinkers and compositions of the invention are useful in any application where a change from a first chemical state to a second chemical state is desired.
Crosslinkers
Compositions of the invention contains degradable crosslinkers, but may include other types of crosslinkers as well. xe2x80x9cDegradable crosslinkersxe2x80x9d are those crosslinkers comprising at least one covalent bond, wherein the covalent bond fragments (or degrades) irreversibly (i.e., the fragments cannot recombine in any manner and they cannot react with any other portion of the polymer in which they are incorporated to form a new covalent crosslink) upon activation. Depending on the exact chemical nature of the degradable crosslinker, fragmentation may occur, for example, by an elimination or cyclization chemical reaction.
At least two fragments remain incorporated as pendant moieties on the polymer in which the degradable crosslinker was incorporated. In some embodiments, however, more than two fragments are produced and not all of the fragments remain incorporated as pendant moieties on the polymer. Nevertheless, the sum of the molecular weights of the total fragments is essentially the same as the molecular weight of the degradable crosslinker prior to fragmentation.
The degradable crosslinkers are generally storage-stable until activation. xe2x80x9cStorage-stablexe2x80x9d degradable crosslinkers and compositions containing degradable crosslinkers are those degradable crosslinkers that remain essentially unfragmented (or crosslinked in the case of compositions containing the degradable crosslinkers) when formed and until activation of the degradable crosslinker (also referred to as xe2x80x9cactivation of the compositionxe2x80x9d).
The shelf life of the degradable crosslinker is generally long enough to permit the use of the degradable crosslinker and compositions therefrom in the desired application. Preferably, the shelf life of the degradable crosslinker is at least about three days, more preferably at least about one month, even more preferably at least about six months, and most preferably at least about one year. That is, the degradable crosslinkers do not prematurely fragment to substantially affect properties of the composition in which they reside.
Activation of the degradable crosslinker occurs by application of an external energy source (e.g., heat or thermal radiation, as well as ultraviolet radiation) to the degradable crosslinker. Typically, and preferably, the external energy source is a thermal energy source (e.g., heat). Thus, preferred degradable crosslinkers are thermally degradable crosslinkers.
xe2x80x9cThermally degradable crosslinkersxe2x80x9d typically fragment according to the invention when subjected to temperatures greater than the polymerization temperature of the polymer in which they are integrated, preferably when subjected to temperatures at least about 20xc2x0 C. greater than the polymerization temperature of the polymer in which they are incorporated. Preferably, the degradable crosslinker fragments at a temperature less than the temperature at which the polymer in which it is integrated degrades (i.e., the temperature at which the polymer becomes useless for its intended purpose, such as by uncontrollably crosslinking or charring).
The temperature at which a thermally degradable crosslinker fragments within about two hours or less is referred to as its xe2x80x9cactivation temperature.xe2x80x9d When incorporated into polymers, the activation temperature of the crosslinked polymer composition is typically the same as the activation temperature of the thermally degradable crosslinker (or the thermally degradable crosslinker having the lowest activation temperature) incorporated therein. Preferably, the activation temperature is about 80xc2x0 C. to about 200xc2x0 C., more preferably about 80xc2x0 C. to about 180xc2x0 C. Compositions containing the degradable crosslinkers are subjected to any suitable temperature to transform the composition from a first chemical state into the desired second chemical state. The length of exposure is adjusted, for example, depending on the proportion of degradable crosslinkers of which fragmentation is desired.
In a further embodiment, in addition to using an external energy source, activation of the degradable crosslinkers can occur by catalysis. Catalysis can accelerate the fragmentation rate of the degradable crosslinker and/or lower the activation temperature of the degradable crosslinker. In catalysis, the catalyst does not become incorporated into the degradable crosslinker or fragments therefrom, but merely acts to accelerate the fragmentation of the degradable crosslinker.
In one embodiment where fragmentation of the degradable crosslinker is catalyzed, an acid or base is added to the degradable crosslinker and compositions containing the same. For example, fragmentation of an aldoxime ester into a nitrile and a carboxylic acid can be facilitated by the addition of either an acid or a base. Bases that can be used to initiate acid/base catalyzed fragmentation include, for example, organic bases such as tertiary amines and mixtures thereof. Acids that can be used to initiate this type of fragmentation include, for example, sulfuric acid, p-toluene sulfonic acid, oxalic acid, and mixtures thereof
Fragmentation of other degradable crosslinkers can also be initiated in this manner or using other appropriate mechanisms as recognizable to those of ordinary skill in the art. The catalyst can be introduced into the composition in either an active or latent state (e.g., a catalyst that does not become active until irradiated with ultraviolet radiation). Generally, the catalyst is added after polymerization of the polymer in which the degradable crosslinker is incorporated. By adding the catalyst after polymerization, one need not be concerned with activation of the degradable crosslinker prior to, or during, formation of the polymer.
For acid-/base-catalyzed fragmentation, any suitable amount of an acid or base is added to the composition in order to accelerate the fragmentation rate of the degradable crosslinker and/or lower the activation temperature of the degradable crosslinker. Then, the mixture is heated to a temperature sufficient to initiate fragmentation of the degradable crosslinker (i.e., the activation temperature). Typically, the activation temperature is lower than those activation temperatures where catalysis is not used, such as those described above with respect to thermal activation of degradable crosslinkers.
Advantageously, degradable crosslinkers fragment, upon activation, into at least two fragments (i.e., those fragments that remain incorporated as pendant moieties on the polymer in which the degradable crosslinker was incorporated) that are essentially free of free radicals and ethylenic unsaturation. Therefore, compositions containing the degradable crosslinkers are relatively stable after transformation to their altered chemical state, as compared to, for example, the compositions of Hendry et al. that contain polymers having fragments containing free radicals or ethylenic unsaturation, both of which are susceptible to recombination or degradation.
Any suitable chemistry can be used for the degradable crosslinker so long as the crosslinker contains at least one energetically labile moiety (i.e., a moiety that fragments upon activation by an external energy source). A degradable crosslinker may contain more than one energetically labile moiety. The energetically labile moieties may or may not be the same throughout the degradable crosslinker.
According to the invention, the fragments that remain incorporated as pendant moieties on the polymer in which the degradable crosslinker was incorporated are essentially free of free radicals and ethylenic unsaturation. Suitable degradable crosslinker chemistries for achieving this advantage include, for example, those containing esters, particularly those containing the following esters and carbonates: aldoxime esters, aldoxime carbonates, amide esters, and mixtures thereof.
Mixtures of various degradable crosslinkers can be used in accordance with the present invention. For example, if a multiple-tier degradation is desired, a mixture of different thermally degradable crosslinkers may be useful. That is, some of the crosslinkers may fragment at lower temperatures than other degradable crosslinkers in the composition. Furthermore, a mixture of degradable crosslinkers that fragment when subjected to different external energy sources (e.g., a mixture containing at least one thermally degradable crosslinker and at least one degradable crosslinker that fragments upon exposure to ultraviolet radiation) can be used in compositions of the invention. The crosslinkers can then be degraded in a stepwise manner, if desired, providing intermediate compositions having increasingly lower levels of crosslinking density and, thus, increasingly different chemical states.
Furthermore, a mixture of degradable crosslinkers and conventional (i.e., non-degradable) crosslinkers may also be used. That is, the degradable crosslinkers as defined in the present invention would contain energetically labile moieties. Conventional crosslinkers, which include all crosslinkers that do not meet the requirements of the present invention, may or may not contain energetically labile moieties. For example, the conventional crosslinkers described by Hendry et al. (U.S. Pat. No. 3,909,497) and thermally reversible crosslinkers (e.g., ionic or physical crosslinkers) may be used in conjunction with degradable crosslinkers of the invention. Compositions containing the crosslinkers could be subjected to an external energy source to activate the degradable crosslinkers, causing fragmentation of all, or a portion, of the degradable crosslinkers. In any event, in certain embodiments, the remaining composition still retains some degree of crosslinking, which is imparted by the conventional crosslinkers.
Each degradable crosslinker typically contains at least two free radically polymerizable groups (e.g., ethylenically unsaturated groups) when the degradable crosslinker is used to crosslink polymer compositions. The free radically polymerizable groups are generally copolymerizable with monomers used to prepare the polymers in the composition. Preferably, the free radically polymerizable groups are part of (meth)acrylate moieties in the degradable crosslinker. By using (meth)acrylate degradable crosslinkers, incorporation of the degradable crosslinkers into (meth)acrylate polymers is facilitated. That is, (meth)acrylate degradable crosslinkers have similar polymerization reactivities as (meth)acrylate monomers, with which they may be copolymerized.
The presence of the free radically polymerizable groups facilitates incorporation of the degradable crosslinker into a polymer during polymerization. By incorporating the degradable crosslinker into the polymer, crosslinking of the polymer results. As free radical polymerization is the preferred polymerization method, the degradable crosslinker preferably does not contain any groups that interfere with (i.e., slow the rate of, or prevent any) free radical polymerization.
The degradable crosslinkers are further described below, with reference to certain terms understood by those in the chemical arts as referring to certain hydrocarbon groups. Such hydrocarbon groups, as used herein, may include one or more heteroatoms (e.g., oxygen, nitrogen, sulfur, or halogen atoms), as well as functional groups (e.g., oxime, ester, carbonate, amide, ether, urethane, urea, carbonyl groups, or mixtures thereof). Depending on the chemistry, the functional groups may also be degradable according to the present invention.
The term xe2x80x9caliphatic groupxe2x80x9d means a saturated or unsaturated, linear, branched, or cyclic hydrocarbon group. This term is used to encompass, alkyl, cycloalkyl, alkylene (e.g., thioalkylene and oxyalkylene), alkenylene, alkenyl, cycloalkenyl, aralkylene, aralkenylene, cycloalkylene, and cycloalkenylene groups, for example.
The term xe2x80x9calkyl groupxe2x80x9d means a saturated, linear or branched, monovalent hydrocarbon group (e.g., a methyl, ethyl, isopropyl, t-butyl, heptyl, dodecyl, octadecyl, amyl, or 2-ethylhexyl group, and the like).
The term xe2x80x9ccycloalkyl groupxe2x80x9d means a saturated, cyclic, monovalent hydrocarbon group.
The term xe2x80x9calkylene groupxe2x80x9d means a saturated, linear or branched, divalent hydrocarbon group. Examples of particular alkylene groups are thioalkylene and oxyalkylene groups.
The term xe2x80x9cthioalkylene groupxe2x80x9d means a saturated, linear or branched, divalent hydrocarbon group with a terminal sulfur atom.
The term xe2x80x9coxyalkylene groupxe2x80x9d means a saturated, linear or branched, divalent hydrocarbon group with a terminal oxygen atom.
The term xe2x80x9calkenylene groupxe2x80x9d means an unsaturated, linear or branched, divalent hydrocarbon group with one or more carbon-carbon double bonds.
The term xe2x80x9calkenyl groupxe2x80x9d means an unsaturated, linear or branched, monovalent hydrocarbon group with one or more carbon-carbon double bonds (e.g., a vinyl group).
The term xe2x80x9ccycloalkenyl groupxe2x80x9d means an unsaturated, cyclic, monovalent hydrocarbon group with one or more carbon-carbon double bonds.
The term xe2x80x9caralkylene groupxe2x80x9d means a saturated, linear or branched, divalent hydrocarbon group containing at least one aromatic group.
The term xe2x80x9caralkenylene groupxe2x80x9d means an unsaturated, linear or branched, divalent hydrocarbon group containing at least one aromatic group and one or more carbon-carbon double bonds.
The term xe2x80x9ccycloalkylene groupxe2x80x9d means a saturated, linear or branched, divalent hydrocarbon group containing at least one cyclic group.
The term xe2x80x9ccycloalkenylene groupxe2x80x9d means an unsaturated, linear or branched, divalent hydrocarbon group containing at least one saturated or unsaturated cyclic group and at least one carbon-carbon double bond.
The term xe2x80x9caromatic groupxe2x80x9d means a mononuclear aromatic hydrocarbon group or polynuclear aromatic hydrocarbon group. The term includes both aryl groups and arylene groups.
The term xe2x80x9caryl groupxe2x80x9d means a monovalent aromatic group.
The term xe2x80x9carylene groupxe2x80x9d means a divalent aromatic group.
Oxime Esters and Oxime Carbonates
Any suitable oxime ester or oxime carbonate can be used so long as it meets the definition of degradable crosslinkers of the invention. Most preferred are degradable crosslinkers containing at least one aldoxime ester moiety, aldoxime carbonate moiety, or mixtures thereof. Aldoxime esters and aldoxime carbonates were found to be more readily thermally degradable as compared to oxime esters and aldoxime carbonates in general.
Aldoxime esters (including, for example, aldoxime esters and aldoxime thioic esters) and aldoxime carbonates (including, for example, aldoxime carbonates, aldoxime xanthates, and aldoxime trithiocarbonates) generally disassociate into a nitrile fragment and an acid fragment and generally conform to the following structure (I): 
R1 is a moiety that contains a free radically polymerizable group (e.g., a (meth)acrylate, (meth)acrylamide, styrene, vinyl ester, or fumarate group). For example, R1 can comprise an alkylene, thioalkylene, oxyalkylene, alkenylene, alkenyl, cycloalkenyl, aralkylene, cycloalkylene, aralkenylene, cycloalkenylene, or arylene group linked to the carbonyl group (including thiocarbonyl group) of the ester or carbonate. When R1 comprises, for example, an oxyalkylene or thioalkylene group linked to the carbonyl group (including thiocarbonyl group, if Y is a sulfur atom) through its respective terminal oxygen or sulfur atom, the degradable crosslinker is an oxime carbonate (including thiocarbonates if Y is a sulfur atom). The R1 moiety may also be substituted with one or more heteroatoms (e.g., O, S, N, halogens, etc.) or functional groups (e.g., oximes, esters, carbonates, amides, ethers, urethanes, ureas, carbonyl groups, or mixtures thereof) in the hydrocarbon chain that are inert to free radical polymerization. The acid corresponding to R1 (i.e., R1C(Y)XH) preferably has a pKa value of greater than one for increased storage stability.
R2 is a moiety that may or may not contain a free radically polymerizable group (e.g., a (meth)acrylate, (meth)acrylamide, styrene, vinyl ester, or fumarate group). Typically, R2 comprises a hydrocarbon group linked to the carbon atom of the aldoxime by a carbon atom. For example, R2 can comprise an alkylene, cycloalkenyl, alkenyl, alkenylene, aralkylene, aralkenylene, cycloalkylene, cycloalkenylene, or arylene group linked to the carbonyl group (including thiocarbonyl group) of the ester or carbonate. The R2 moiety may also be substituted with one or more heteroatoms (e.g., O, S, N, halogens, etc.) or functional groups (e.g., oximes, esters, carbonates, amides, ethers, urethanes, ureas, carbonyl groups, or mixtures thereof) in the hydrocarbon chain that are inert to free radical polymerization.
X and Y are independently selected from oxygen (O) and sulfur (S), depending on the type of aldoxime ester or aldoxime carbonate.
The following examples are provided based on the assumption that R1 is linked to the carbonyl (or thiocarbonyl) group of the ester (or carbonate) by a carbon atom. If R1 terminates in an oxygen or sulfur atom, however, nomenclature would be adjusted as understood by those of ordinary skill in the art. When X is oxygen and Y is oxygen, the aldoxime ester is an aldoxime ester. When X is sulfur and Y is oxygen, the aldoxime ester is an aldoxime thioic S-ester. When X is oxygen and Y is sulfur, the aldoxime ester is an aldoxime thioic O-ester. When X and Y are both sulfur, the aldoxime ester is an aldoxime thioic ester.
The subscript xe2x80x9cnxe2x80x9d is an integer of one or greater. When n is one, R2 contains a free radically polymerizable group. When n is greater than one, R2 may or may not contain a free radically polymerizable group.
Particularly preferred aldoxime esters and carbonates in the class include those where R1 is independently selected from the following moieties: 
and 
and those where R2 is independently selected from the following moieties: 
where a is 3, 4, 5, or 6, 
Further preferred embodiments of degradable crosslinkers of the invention are those where n is 2 and R1 is selected from: 
and R2 is: 
The aldoxime esters and carbonates can be prepared using any suitable method as recognizable to those of ordinary skill in the art. Aldehydes are a general starting material for preparation of the aldoxime esters and carbonates. For example, difunctional (meth)acrylate (i.e., methacrylate or acrylate) aldoxime esters and carbonates can be prepared from diethers of salicylaldehyde. The aldehyde either contains a free radically polymerizable group or is linked to a free radically polymerizable group through a functional group in the aldehyde. During preparation of the aldoxime esters and carbonates, aldehydes are converted to aldoximes with hydroxylamine.
The aldoximes are generally esterified with an acid chloride containing a free radically polymerizable group to make aldoxime esters. Those of ordinary skill in the art will understand that the reactants and/or reaction sequence can be varied to arrive at the same products.
Alternatively, when making aldoxime carbonates, the aldoxime is reacted with a phosgene instead of esterifying the aldoxime. The reaction product is then further reacted with an alcohol to form an aldoxime carbonate. Alternatively, the aldoxime is reacted with a chloroformate instead of esterifying the aldoxime to arrive at an aldoxime carbonate. Again, those of ordinary skill in the art will understand that the reactants and/or reaction sequence can be varied to arrive at the same products.
Subsequent reaction with multifunctional ethylenically unsaturated compounds, such as (meth)acryloyl chlorides, to impart additional free radically polymerizable groups to the aldoxime ester or carbonate, may then be performed, if desired. Specific preparation examples are provided in the Examples section, infra.
Amide Esters
An amide ester, as used herein, is a compound containing at least one amide group and at least one ester group. Amide esters (including, for example, amide esters, amide thioic esters, and amide dithioic esters) generally cyclize to an imide, eliminating an alcohol or thiol and generally conform to the following structure (II): 
R3 is a moiety that contains a free radically polymerizable group (e.g., a (meth)acrylate, (meth)acrylamide, styrene, vinyl ester, or fumarate group) and links to the oxygen atom of the ester (if X and Y1 are both oxygen) or thioic O-ester (if X is oxygen and Y1 is sulfur), or links to the sulfur atom of the thioic S-ester (if X is sulfur and Y1 is oxygen) or dithioic ester (if X and Y1 are both sulfur). For example, R3 can comprise an alkenylene, alkenyl, cycloalkenyl, alkylene, aralkylene, aralkenylene, cycloalkylene, cycloalkenylene, or arylene group linked to the oxygen atom of the ester (if X and Y1 are both oxygen), thioic O-ester (if X is oxygen and Y1 is sulfur), thioic S-ester (if X is sulfur and Y1 is oxygen), or dithioic ester (if X and Y1 are both sulfur). The R3 moiety may also be substituted with one or more heteroatoms (e.g., O, S, N, halogens, etc.) or functional groups in the hydrocarbon chain (e.g., oximes, esters, carbonates, amides, ethers, urethanes, ureas, carbonyl groups, or mixtures thereof) that are inert to free radical polymerization.
R4 is a moiety that may or may not contain a free radically polymerizable group (e.g., a (meth)acrylate, (meth)acrylamide, styrene, vinyl ester, or fumarate group). For example, R4 can comprise any hydrocarbon group linked to the nitrogen atom of the amide by a carbon atom. For example, R4 can comprise an alkyl, cycloalkyl, alkylene, thioalkylene, oxyalkylene, alkenylene, alkenyl, cycloalkenyl, aralkylene, aralkenylene, cycloalkylene, cycloalkenylene, aryl, or arylene group linked to the nitrogen atom of the amide. The R4 moiety may also be substituted with one or more heteroatoms (e.g., O, S, N, halogens, etc.) or functional groups (e.g., oximes, esters, carbonates, amides, ethers, urethanes, ureas, carbonyl groups, or mixtures thereof) in the hydrocarbon chain that are inert to free radical polymerization.
R5 is a moiety that links to the amide carbonyl (if Y1 is oxygen) or amide thiocarbonyl (if Y2 is sulfur) with at least one carbon atom and to the ester carbonyl (if Y1 is oxygen) or ester thiocarbonyl (if Y2 is sulfur) with at least one carbon atom. R5 may or may not contain a free radically polymerizable group (e.g., a (meth)acrylate, (meth)acrylamide, styrene, vinyl ester, or fumarate group). However, one, but not both, of R4 or R5 contains a free radically polymerizable group. Examples of R5 include those moieties capable of becoming incorporated into a 5- or 6-membered ring, which ring includes R5 and the imide, upon cyclization of the degradable crosslinker to an imide. As such, typically R5 contains two or three atoms in a chain, with the terminal atoms each being carbon. When there are three atoms in the chain, the center atom may be, for example, carbon or a divalent heteroatom (e.g., oxygen or sulfur). The chain may also contain pendent hydrocarbon groups therefrom, such as when R5 contains a free radically polymerizable group. One or more atoms in the chain may also be part of a ring structure. Preferably, R5 comprises an alkylene or arylene (e.g., ortho arylene) group.
X, Y1, and Y2 are independently selected from oxygen (O) and sulfur (S), depending on the type of amide ester. That is, each Y constituent (Y1 and Y2) occurring within an amide ester may be the same or different from other Y constituents. Similarly, X may be the same as one or both of the Y constituents. Alternatively, X may be different from each of the Y constituents.
When X is oxygen, Y1 is oxygen, and Y2 is oxygen, the amide ester is an amide ester. When X is oxygen, Y1 is oxygen, and Y2 is sulfur, the amide ester is a thioamide ester. When X is oxygen, Y1 is sulfur, and Y2 is oxygen, the amide ester is an amide thioic O-ester. When X is oxygen, Y1 is sulfur, and Y2 is sulfur, the amide ester is a thioamide thioic O-ester.
When X is sulfur, Y1 is sulfur, and Y2 is sulfur, the amide ester is a thioamide dithioic ester. When X is sulfur, Y1 is oxygen, and Y2 is sulfur, the amide ester is a thioamide thioic S-ester. When X is sulfur, Y1 is sulfur, and Y2 is oxygen, the amide ester is an amide dithioic ester. When X is sulfur, Y1 is oxygen, and Y2 is oxygen, the amide ester is an amide thioic S-ester.
Particularly preferred amide esters in the class include those where R3 is: 
R4 is:
xe2x80x94[CH2]2xe2x80x94CH3
xe2x80x83and R5 is: 
The amide esters can be prepared using any suitable method as recognizable to those of ordinary skill in the art. Anhydrides are a general starting material for preparation of the amide esters. The anhydride is generally combined with a free radically polymerizable alcohol to form an ester acid. The acid moiety is then converted to a more chemically reactive derivative, such as an acid chloride, that can react with a primary amine, such as an amino alcohol or diamine. Upon reaction with the primary amine, an amide ester is formed. Subsequent reaction with multifunctional ethylenically unsaturated compounds, such as (meth)acryloyl chlorides, to impart additional free radically polymerizable groups to the amide ester may then be performed, if desired. Those of ordinary skill in the art will understand that the reactants and/or reaction sequence can be varied to arrive at the same products. Specific preparation examples are provided in the Examples section, infra.
Preparation of Crosslinked Polymer Compositions Comprising the Degradable Crosslinker
The degradable crosslinkers can be used in a wide variety of polymers. Depending on the amounts and types of components in the crosslinked polymer compositions, the crosslinked polymer compositions can be tacky to the touch or essentially non-tacky to the touch at room temperature.
Generally, the degradable crosslinker is copolymerized with the monomer component (i.e., one or more monomers that are copolymerizable with the degradable crosslinker) used to prepare the polymers. The monomer component and degradable crosslinker are copolymerized according to any suitable method, as recognizable to those of ordinary skill in the art.
Any suitable monomer, or combination thereof, may be used. Preferably, monomers of the present invention are ethylenically unsaturated monomers, preferably monoethylenically unsaturated monomers, such that they can be copolymerized with the degradable crosslinkers. Preferably, the monomers are selected from (meth)acrylates, (meth)acrylic acids, vinyl esters, (meth)acrylamides, and combinations thereof.
Particularly preferred monomers are (meth)acrylate monomers, including monoethylenically unsaturated monomers, such as (meth)acrylate esters of non-tertiary alkyl alcohols, the alkyl groups of which comprise from about 1 to about 18 carbon atoms, preferably about 4 to about 12 carbon atoms, and mixtures thereof Examples of suitable (meth)acrylate monomers useful in the present invention include, but are not limited to, methylacrylate, ethylacrylate, methyl methacrylate, ethyl methacrylate, n-butyl acrylate, decyl acrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, hexyl acrylate, isoamyl acrylate, isodecyl acrylate, isodecyl methacrylate, isononyl acrylate, isooctyl acrylate, lauryl acrylate, 2-methyl butyl acrylate, 4-methyl-2-pentyl acrylate, ethoxyethoxyethyl acrylate, isobornyl acrylate, isobornyl methacrylate, 4-t-butylcyclohexyl methacrylate, cyclohexyl methacrylate, phenyl acrylate, phenylmethacrylate, 2-naphthyl acrylate, 2-naphthyl methacrylate, and mixtures thereof. Particularly preferred are 2-ethylhexyl acrylate, isooctyl acrylate, lauryl acrylate, n-butyl acrylate, ethoxyethoxyethyl acrylate, and mixtures thereof.
Examples of other ethylenically unsaturated monomers include, but are not limited to, vinyl esters (e.g., vinyl acetate, vinyl pivalate, and vinyl neononanoate); vinyl amides; N-vinyl lactams (e.g., N-vinyl pyrrolidone and N-vinyl caprolactam); (meth)acrylamides (e.g., N,N-dimethyl acrylamide, N,N-dimethyl methacrylamide, N,N-diethyl acrylamide, and N,N-diethyl methacrylamide); (meth)acrylonitrile; -maleic anhydride; styrene and substituted styrene derivatives (e.g., xcex1-methyl styrene); and mixtures thereof.
Optional acidic monomers may also be used. Useful acidic monomers include, but are not limited to, those selected from ethylenically unsaturated carboxylic acids, ethylenically unsaturated sulfonic acids, ethylenically unsaturated phosphonic acids, and mixtures thereof Examples of such compounds include those selected from acrylic acid, methacrylic acid, itaconic acid, fumaric acid, crotonic acid, citraconic acid, maleic acid, xcex2-carboxyethyl acrylate, 2-sulfoethyl methacrylate, styrene sulfonic acid, 2-acrylamido-2-methylpropane sulfonic acid, vinyl phosphonic acid, and the like, and mixtures thereof.
The following polymerization techniques, which are described infra, can be used for polymerizing the monomer component and degradable crosslinker. These techniques include, but are not limited to: the conventional techniques of solvent polymerization, dispersion polymerization, emulsion polymerization, suspension polymerization, solventless bulk polymerization, and radiation polymerization, including processes using ultraviolet light, electron beam, and gamma radiation. The starting materials may comprise any suitable additives, such as a polymerization initiator, especially a thermal initiator or a photoinitiator, of a type and in an amount effective to polymerize the monomers and degradable crosslinker.
When polymerized using emulsion, suspension, or dispersion polymerization, the resulting crosslinked polymer compositions are advantageously capable of being readily made into storage-stable, free-flowing polymer compositions. xe2x80x9cFree-flowingxe2x80x9d polymer compositions are those compositions containing a multitude of solid state particulates that do not substantially agglomerate (i.e., the particulates are capable of being moved by gravitational forces alone) at temperatures below the activation temperature of the degradable crosslinkers. Preferably, the free-flowing polymer compositions do not substantially agglomerate at the storage temperature and pressure.
The free-flowing polymer compositions are especially useful in certain feeding, blending, and delivery methods. For example, free-flowing polymer compositions facilitate feeding hot-melt coating compositions. Conventional feeding equipment, such as hopper feeders and powder conveyers, can be used with free-flowing polymer compositions of the invention. Furthermore, shipping and handling of free-flowing polymer compositions is typically less expensive and more convenient than shipping and handling associated with bulk polymer compositions. Free-flowing polymer compositions can also be easily dry-blended, without the need for solvents or melt blending.
The free-flowing polymer compositions can be prepared using any suitable method. In one embodiment, the particulates are filtered to a dryness of about 75% to about 95% solids using any suitable filtering technique, such as using a Nxc3x9cTSCHE filter (commercially available from Northland Stainless, Inc.; Tomahawk, Wis.). The filtered particulates are then blended with silica (such as that commercially available from Degussa Corporation; Ridgefield Park, N.J., under the trade designation, AEROSIL R-972), using any suitable blending technique, such as using a ribbon blender. The coated particulates are then dried using any suitable technique. For example, the coated particulates can be air-dried in an oven. The coated particulates can also be air-dried in a fluid bed dryer, such as those commercially available from Glatt Air Techniques Inc.; Ramsey, N.J.
Solvent Polymerization Method
Solvent polymerization is well known in the art and described in various sources such as U.S. Pat. No. Re 24,906 (Ulrich) and U.S. Pat. No. 4,554,324 (Husman et al.). Briefly, the procedure is carried out by adding the monomers, degradable crosslinker, a suitable solvent such as ethyl acetate, and an optional chain transfer agent to a reaction vessel. Then, a free radical initiator is added to the mixture. Many suitable free radical initiators are commercially available, such as those available from E. I. duPont de Nemours and Company; Wilmington, Del. under the VAZO trade designation. Specific examples include VAZO 64 and VAZO 52, which are described below in the Table of Abbreviations. Suitable initiators also include hydroperoxides, such as tert-butyl hydroperoxide, and peroxides, such as benzoyl peroxide and cyclohexane peroxide.
After purging with nitrogen, the reaction vessel is maintained at an elevated temperature, typically in the range of about 40xc2x0 C. to about 100xc2x0 C., until the reaction is complete. Typically, the reaction takes about one to about twenty hours to complete, depending on the batch size and reaction temperature.
Dispersion Polymerization Method
Dispersion polymerization typically is carried out as single-phase reaction of a mixture consisting of a solution of monomers, degradable crosslinker, initiator, and steric stabilizer in a solvent that does not dissolve the resulting polymer. The initial stage of the polymerization is a typical solution polymerization and the polymer chains grow in size until they become insoluble in the reaction mixture. As the polymer starts to precipitate out of the mixture, the steric stabilizer adsorbs on the surface of the polymer, preventing coalescence of the polymer particles as they form. The reaction will continue until all of the monomer is consumed, resulting in the formation of crosslinked polymer particles that are insoluble in the reaction medium in which they are formed.
Emulsion Polymerization Method
Emulsion polymerization is also described in U.S. Pat. No. Re 24,906 (Ulrich). For example, the monomers are mixed with the degradable crosslinker, distilled water, an emulsifying agent, and suitable initiators in a reaction vessel. Then, the reaction vessel is purged with nitrogen and heated, with agitation, to a temperature in the range of about 25xc2x0 C. to about 80xc2x0 C., until the reaction is complete.
For ease of handling and/or to employ emulsion-polymerized polymers as powders, the emulsion-polymerized polymer is typically spray-dried using conventional drying techniques. To prepare such powders, the emulsion-polymerized base copolymer can be fed to a nozzle that sprays the emulsion into a stream of hot gas. The aqueous emulsion medium evaporates first, forming a small droplet of concentrated polymer. As aqueous medium removal nears completion, the droplet is transformed into a powder particle or clusters thereof See, for example, U.S. Pat. No. 3,772,262 (Clementi) and K. Masters, xe2x80x9cSpray Drying,xe2x80x9d 2nd ed., Wiley: 1976.
Suspension Polymerization Method
Crosslinked polymer compositions can also be prepared in bead form using suspension polymerization methods. Such suspension methods are described, for example, in European Patent Application Number 0 853 092 (Minnesota Mining and Manufacturing Co.). This suspension process involves mixing the monomers, degradable crosslinker, free radical initiator, chain transfer agent, and other desired additives to form a premix. A suspension stabilizer, such as dextrin or a dextrin derivative, is combined with water and then with the premix to form an oil-in-water suspension. The resulting suspension typically comprises from about 10 to about 50 weight percent premix and from about 90 to about 50 weight percent water phase.
Polymerization is then initiated, typically thermally, and carried out for about two to about sixteen hours at a temperature of about 40xc2x0 C. to about 90xc2x0 C. The crosslinked polymer beads can be isolated by a variety of means and generally have a diameter of one to 5,000 micrometers. Similar to the emulsion process, smaller suspension-polymerized particles may also be capable of being spray-dried to recover the polymer. Larger particles may be capable of being isolated, for example, by simple filtration and air-drying.
In certain embodiments, after isolating from suspension and drying, the suspension beads can show some blocking resulting from tackiness of the suspension beads. Due to this blocking, the suspension beads can partially, or even completely, lose desired free-flowing characteristics. To prevent this loss of flow, a dusting agent, such as hydrophobic silica (e.g., AEROSIL R-972, commercially available from Degussa Corporation; Ridgefield Park, N.J.), can be added immediately after isolating the beads. When treated in this manner, suspension beads can also be used in processing methods and delivery techniques that advantageously exploit the powder-like nature of these materials.
Solventless Polymerization Method
Solventless polymerization methods, such as the methods described for polymerizing packaged polymerizable compositions in U.S. Pat. No. 5,804,610 (Hamer et al.), may also be utilized to prepare the crosslinked polymer compositions. In one embodiment, from about 0.1 to about 500 grams of a polymerizable mixture comprising monomers, degradable crosslinker, initiator, and optional chain transfer agent is completely surrounded by a packaging material. In another preferred embodiment, the polymerizable mixture is disposed on the surface of a sheet, or between a pair of two substantially parallel sheets, of the packaging material.
The packaging material is made of a material that, when combined with the resulting polymer, does not substantially adversely affect the desired polymer characteristics. The packaging material should be appropriate for the polymerization method used. For example, with photopolymerization, it is necessary to use a film material that is sufficiently transparent to ultraviolet radiation at the wavelengths necessary to effect polymerization. Polymerization can be effected by exposure to ultraviolet (UV) radiation as described in U.S. Pat. No. 4,181,752 (Martens et al.). In a preferred embodiment, the polymerization is carried out with UV black lights having over 60 percent, and preferably over 75 percent, of their emission spectra between about 280 to about 400 nanometers (nm), with an intensity between about 0.1 to about 25 mW/cm2.
In another solventless polymerization method, crosslinked polymer compositions of the present invention are prepared by photoinitiated polymerization methods according to the technique described, for example, in U.S. Pat. No. 4,181,752 (Martens et al.). For example, the monomers and photoinitiator can be mixed together in the absence of solvent and partially polymerized to make a coatable syrup. Coatable syrups generally have a viscosity in the range of from about 500 centiPoise to about 50,000 centiPoise. In yet another way, the monomers can be mixed with a thixotropic agent, such as fumed hydrophilic silica, to achieve a thickened monomer mixture having a coatable thickness. The degradable crosslinker and any other ingredients are then added to the prepolymerized syrup or thickened monomer mixture. Alternatively, these other ingredients (preferably, with the exception of the degradable crosslinker) can be mixed with the monomers prior to partial polymerization or thickening of the monomer mixture.
The resulting polymerizable composition is coated onto a substrate (which may be transparent to UV radiation) and polymerized in an inert atmosphere (i.e., an oxygen-free, such as nitrogen, atmosphere) by exposure to UV radiation. A sufficiently inert atmosphere can also be achieved by covering a layer of the polymerizable coating with a plastic film that is substantially transparent to UV radiation and irradiating through that film, as described in U.S. Pat. No. 4,181,752 (martens et al.), using a UV light source. Alternatively, instead of covering the polymerizable coating, an oxidizable tin compound may be added to the polymerizable composition to increase the tolerance of the composition to oxygen, as described in U.S. Pat. No. 4,303,485 (Levens). The UV light source preferably has 90% of the emissions between about 280 and about 400 nanometers (more preferably between about 300 and about 400 nanometers), with a maximum emission at about 350 nanometers.
Additives
A wide variety of conventional additives can be mixed with the crosslinked polymer composition. In fact, when the crosslinked polymer composition is essentially non-tacky at room temperature, blending of additives with the crosslinked polymer composition is often easier. The components (i.e., crosslinked polymer composition and additives) are even able to be dry-blended, as opposed to the more costly and complicated melt-blending techniques.
Any suitable additive can be blended with the crosslinked polymer composition. For certain applications, expandable microspheres, glass bubbles, and chemical blowing agents may be useful additives. Those of ordinary skill in the art will recognize a wide variety of additives that may be useful when preparing crosslinked polymer compositions of the invention for specific applications.
For example, tackifiers can be added to the crosslinked polymer composition to increase the composition""s tack. Plasticizers can also be added to the crosslinked polymer composition. For example, when the polymer is derived from a high proportion of relatively high glass transition temperature (Tg) monomers, addition of a plasticizer can increase the tack of the composition.
If an increase in crosslink density of the composition is desired at any time after fragmentation of at least a portion of the degradable crosslinkers, crosslinking additives may be added to the composition. For example, ultraviolet (UV) crosslinkers facilitate crosslinking by exposure to ultraviolet radiation. Thermally reversible crosslinkers, such as those described in PCT Publication Number WO 99/42,536 (Minnesota Mining and Manufacturing Co.), facilitate crosslinking without requiring an external energy source, such as radiation.
If the crosslinked polymer composition is tacky to the touch at ambient temperature and pressure, it may be desirable to use a coating agent, such as the shell materials described in U.S. Pat. No. 5,322,731 (Callahan, Jr. et al.), or a dusting agent, such as hydrophobic silica or the like, in order to facilitate easier handling of the crosslinked polymer compositions prior to their application to a substrate. Preferably, when using a coating agent or dusting agent for this purpose, the amount of the coating/dusting agent used is an effective amount to render the composition non-tacky to the touch at ambient temperature and pressure.
Application/Processing of the Crosslinked Polymer Compositions
The crosslinked polymer compositions can be applied to a wide variety of substrates to form a coating thereon. The substrate can take any suitable form, such as, for example, a sheet, a fiber, or a shaped article. The coating thickness will vary depending upon various factors such as, for example, the particular application, the crosslinked polymer composition formulation, and the nature of the substrate (e.g., its absorbency, porosity, surface roughness, amount of creping, chemical composition, etc.). For example, a porous or rough substrate will typically require a thicker coating than less porous or smoother substrates. As another example, pressure-sensitive adhesive coatings typically have a thickness of about 25 microns to about 250 microns.
The method of processing the composition, or applying it to a substrate, will vary depending on the desired use of the composition. In one embodiment, activation of the composition occurs prior to applying the composition to a substrate. In another embodiment, activation of the composition occurs after applying the composition to a substrate. In another embodiment, the composition is activated while it is being applied to a substrate.
To activate the composition (fragmenting at least a portion of the degradable crosslinker, also referred to as xe2x80x9cactivating the degradable crosslinkerxe2x80x9d or a similar phrase), an external energy source is applied to at least a portion of the composition. The energy source used to activate the composition can be diffuse, so as to activate broad areas of the composition, or focused, so as to activate discrete, predetermined portions of the composition. Any suitable energy source can be used, such as a thermal source (e.g., oven, infrared radiation lamp or laser, slot burner, or microwave source (when used in conjunction with a receptor for microwave radiation)).
In one embodiment, the external energy source is applied to a discrete portion of the composition, thereby activating the degradable crosslinker in only a portion of the composition. Resulting compositions may, thus, have varying chemical states throughout, depending on whether the degradable crosslinker was activated in that particular portion. In another embodiment, the external energy source is applied to substantially all of the composition, thereby activating the degradable crosslinker in substantially all of the composition. Resulting compositions, thus, having substantially identical chemical properties throughout.
Activation Followed by Application
In certain embodiments, the crosslinked polymer composition is first activated (to fragment at least a portion of the degradable crosslinker) and then applied to a substrate in separate processing steps. That is, the crosslinked polymer composition is first transformed to a composition having reduced crosslink density by fragmenting at least a portion of the degradable crosslinkers incorporated therein. The composition is then applied to a substrate using any suitable method. For example, organic solvent coating and hot-melt coating techniques, which are described infra, may be used.
Any conventional coating technique can be used to apply the compositions to target substrates from organic solvent solutions. Useful coating techniques include brush, roll, spray, spread, wire, gravure, transfer roll, air knife, or doctor blade coating.
Hot-melting coating techniques are also useful. For example, the compositions may be introduced into a vessel to melt and activate the composition. The composition is subjected to a temperature sufficient to melt and activate the composition and thoroughly mix any additional components, after which the composition is coated onto a substrate. This step can be done conveniently in a heated extruder, bulk tank melter, melt-on-demand equipment, or hand-held hot-melt adhesive gun.
The hot-melt compositions can be coated onto a substrate using any suitable method. For example, the compositions can be delivered out of a film die and coated by contacting the drawn hot-melt composition with a moving web (e.g., plastic web) or other suitable substrate. Using this method, the hot-melt material is applied to the moving preformed web using a die having flexible die lips, such as a rotary rod die. A related coating method involves extruding the composition and a coextruded backing material from a film die and cooling the layered product to form a multi-layered construction, such as an adhesive tape. After forming by any of these continuous methods, the resulting films or constructions can be solidified by quenching using both direct methods (e.g., chilled rolls or water baths) and indirect methods (e.g., air or gas impingement).
Activation with Application
In other embodiments, the crosslinked polymer composition is activated (to fragment at least a portion of the degradable crosslinker) while applying the composition to a substrate in essentially a single processing step.
For example, the composition may be activated and applied using a reactive flame spray technique. The flame spray technique is a process whereby the deposition of a largely molten material is enabled by using a highly directional gas stream. Flame spray methods include flame spraying and reactive flame spraying. Flame spraying involves the introduction of a material to be deposited into a flame that is typically generated external to the gas introduction apparatus. The composition is activated while applying it to a substrate by using reactive flame spraying, wherein the composition, which is typically in the form of a powder, is at least partially activated during spraying.
In flame spray techniques, the kinetic energy of the feed gases typically transports the molten material in a transport gas stream. However, an additional transport gas stream may be utilized. The thermal energy in a flame spray system is determined by the flow rates and composition of fuel and oxidizer gases that are used. Flames are typically created from acetylene and oxygen in order to produce a relatively hot flame. Lower temperature systems, for the deposition of thermally sensitive materials, can be provided by using propane/air flames.
All material introduction methods typically have the same ultimate goalxe2x80x94to produce small, potentially liquified particles that collide with the receiving surface. Commercial flame spray systems are available that use either particle or wire feeds. Other systems have been described that utilize liquid feeds (K. A. Gross, et al, Journal of Thermal Spray Technology, 8, 583-589 (1999). For example, several flame deposition systems have been reported that describe the spraying of a liquid in, or near, a flame to achieve deposition. Material to be deposited in powder form is typically introduced to a depositing flame in a pressurized transport gas stream. This transport gas stream is typically either the oxidizer for the flame or a designated transport gas stream. Powders can be gravity fed or mechanically fed into the transport gas stream. Alternatively, powders can be drawn into a transport gas stream from a contained, fluidized bed due to the venturi effect of the transport gas stream.
Depositing material typically reaches only a fraction of the flame temperature during the flame residence time. Control of this residence time is a key process variable in flame spray deposition. The precise residence time needed for a given system will be determined by the characteristics of the flame spray system and the depositing material. This residence time can be modified, for example, by changing the flow rate of a designated transport gas stream or by changing the precise point of introduction of the depositing material into the flame.
Depositing material may also be modified by further exposure to the flame. Likewise, a surface to be coated may also be modified, such as by heating with a flame or other thermal source prior to deposition, in order to allow the depositing material to remain in a softened, or fluid, state for a longer time on the receiving surface.
Flames generated in typical flame spray systems are viewed exclusively as sources of thermal energy to provide a phase change of the depositing material, such as from a solid particle to a molten droplet. Accompanying this phase change is a change in the rheological properties of the material. Incidental oxidation of the depositing material may also occur in typical flame spray processes. However, reactive flame spraying utilizes the thermal energy of the flame to fragment at least a portion of the degradable crosslinker, which allows the particles to flow more readily when they impinge the substrate and also increases the pressure-sensitive adhesive tack of the applied composition. This fragmentation is a result of a chemical reaction that occurred due to the thermal energy. That is, a flame spray process that specifically utilizes the flame to fundamentally alter the composition of the injected material is considered reactive flame spraying.
Application Followed by Activation
In yet other embodiments, the crosslinked polymer composition is first applied to a substrate and then activated (to fragment at least a portion of the degradable crosslinker). The composition can be applied to a substrate using any suitable method. For example, aqueous solution coating, aqueous dispersion coating, solventless coating (followed by polymerization), thermal deposition, and powder coating can be used.
Any conventional coating technique can be used to apply the compositions to target substrates from aqueous solutions, including emulsions and dispersions. Useful coating techniques include brush, roll, spray, spread, wire, gravure, transfer roll, air knife, curtain, slurry, or doctor blade coating.
As another example, the composition can be applied to a substrate using the solventless coating and polymerization method described in U.S. Pat. No. 4,181,752 (Martens et al.), supra. In this embodiment, the degradable crosslinker is added to a partially polymerized, or thickened, monomer mixture. The resulting composition is then coated onto a substrate and polymerized. After the composition is polymerized, it is activated.
Yet another example of this embodiment involves thermal deposition of the crosslinked polymer composition. Particles of the composition are coated on a substrate and then heated using a suitable thermal source, such as those described in PCT Publication No. WO 99/03,642 (Minnesota Mining and Manufacturing Co.). Optionally, the substrate on which the heated particles are coated is softened to facilitate adhesion of the particles to the substrate. If softened, the substrate is generally softened by thermal energy or thermal radiation. For example, the substrate can be softened using heat from the same thermal source that is used for coating and heating particles of the composition.
A still further example of Application Followed by Activation involves conventional, as opposed to reactive, flame spraying of the crosslinked polymer composition. The conventional flame spray technique was described above with respect to xe2x80x9cActivation with Application.xe2x80x9d
One of the advantages of using crosslinked polymer compositions of the present invention is the ability to deliver the crosslinked polymer composition using powder coating techniques. In addition to the spray-dried, emulsion-polymerized compositions and the free-flowing, suspension- or dispersion-polymerized particles described above, the crosslinked polymer composition in powdered form can also be prepared using mechanical techniques such as cryo-grinding or hammer milling.
The capability of delivering the crosslinked polymer compositions as powdered coatings offers several advantages and alternatives over crosslinked polymer compositions coated using conventional techniques. For example, when the crosslinked polymer composition is in the form of particulates, the crosslinked particulates can be coated on a substrate in the Z-direction (i.e., the direction perpendicular to the substrate) using powder coating techniques, described infra, and then activated to fragment at least a portion of the degradable crosslinkers. Upon fragmentation, the particulates fuse together to form a continuous coating on the substrate. By applying the crosslinked polymer composition to a substrate in this manner, the resulting coating is less oriented, and thus has reduced stress, as compared to coatings applied by conventional hot-melt coating methods, such as die-extrusion. These process-related effects can compromise the performance of the resulting coatings.
Hot-melt coating also has its limitations when three-dimensional, or rough, surface coverage is desired. Furthermore, hot-melt coating of wide sheets, or substrates, requires bulky and expensive custom coating dies that are not readily available. By using powder coating techniques to coat all or a portion of such substrate surfaces with crosslinked polymer compositions of the present invention, all of these disadvantages associated with hot-melt coated compositions can be avoided.
Yet, another advantage of powder format of the crosslinked polymer composition is the ease of blending the crosslinked polymer composition with other powdered components. The other powdered components may be other crosslinked polymer compositions of the current invention with different chemistries or crosslinking densities. Alternatively, the other powdered components can be other organic (e.g., polymeric materials) or inorganic materials.
Useful techniques for powder coating of the crosslinked polymer compositions include fluidized-bed coating and electrostatic spray processes. In the fluidized-bed coating process, the crosslinked polymer composition is placed in a container having a porous plate as its base. Air is passed through the plate, causing the composition to expand in volume and fluidize. In this state, the composition possesses some of the characteristics of a fluid. A substrate is then heated in an is oven to a temperature above the melting point of the crosslinked polymer composition. The substrate is dipped into the fluidized bed where the fluidized composition melts on the substrate to form a coating. Depending on the rheology and crosslink density of the crosslinked polymer composition, activation will either fuse the coating into a smooth coating or all or part of the particulate nature of the crosslinked polymer composition can be maintained. Alternatively, a cold substrate can be run over a bed of fluidized particles that are tribo-charged and, thus, cling to the substrate. The coated substrate can then be passed through a heated zone, or nip, to fuse the coating.
In the electrostatic spray process, the powdered material is dispersed in an air stream and passed through a corona discharge field where the particles acquire an electrostatic charge. The charged particles are attracted to and deposited on the grounded substrate. The substrate, usually electrostatically coated at room temperature, is then placed in an oven where the powder melts and forms a coating. See for example, Kirk-Othmer Encyclopedia of Chemical Technology, 4th Edition, Wiley: 1993, Vol. 6, pages 635-636.
In all embodiments of application/processing of the crosslinked polymer composition, further processing steps may be performed after application and activation of the crosslinked polymer composition. For example, if desired, the activated polymer composition can be recrosslinked at any time after fragmentation of the degradable crosslinker, such as after applying the composition to a substrate. Typically, such xe2x80x9crecrosslinkingxe2x80x9d is accomplished by radiation (e.g., using electron beam or ultraviolet radiation) of the composition or the incorporation of thermally reversible crosslinkers in the composition, such as those described in PCT Publication Number WO 99/42,536 (Minnesota Mining and Manufacturing Co.). A wide variety of other processing steps may also be performed on the resulting polymer compositions.